1. Field of the Invention
The present invention relates generally to an integrated system for generating oxygen and absorbing carbon dioxide. More particularly, the invention relates to an oxygen generating and carbon dioxide absorption method and apparatus that may be used as part of a self contained power supply system for autonomous vehicles.
2. Related Art
Conventional applications for vehicles and devices operating where sufficient quantities of oxygen cannot be directly extracted from the operating atmosphere, and which require control of carbon dioxide and/or sulfur emissions in the operating atmosphere, do not provide for the use of hydrocarbon fuels to provide mechanical, electrical, and/or thermal energy. Such applications may include, but are not limited to, closed environments such as those found on manned and unmanned autonomous vehicles (AVs). Examples of AVs include, for example, autonomous underwater vehicles, space exploration vehicles, or any other vehicles or devices operating in oxygen poor or otherwise adversely contaminated atmospheres, such as robots or remote control devices for exploring toxic, dangerous or environmentally contaminated atmospheres or oxygen limited environments. The invention is not limited to use in unmanned vehicles, but may also be employed in manned vehicles, particularly where a closed atmosphere is required and/or size constraints are present.
Unmanned vehicles that are released from a host vehicle to perform an autonomous mission contain sensor, navigation, communication and robotic systems worth large amounts of money. Reliable power is required to ensure that the mission is accomplished and the vehicle and its valuable cargo is recovered.
Energy system technology may be the most limiting factor for vehicle operating endurance, size, speed, and, ultimately, cost. Conventional energy systems available for powering AVs include rechargeable lithium polymer ion batteries. However, these systems do not possess a sufficiently high power density. Lithium thionyl chloride batteries may also be used; however, these are not rechargeable, are very expensive and pose undo environmental risks.
Leading candidates for AV power technologies involve the application of fuel cells and micro-turbines. Both fuel cells and micro-turbines are at varied stages of development by both commercial and military organizations. The use of other power generating means, for example Brayton cycle engines and Sterling engines, is also contemplated. There is a continuing need for technological developments in fuel cell, micro-turbine and other energy generating systems to significantly increase power and energy densities and expand the use of AVs.
Common to many of the power and energy systems that may be employed in AVs are the requirements for oxygen generation and for byproduct management. Oxygen is typically needed for the combustion of fuel, whether that fuel is a liquid hydrocarbon, a gas such as, for example, hydrogen, or a solid oxide, and whether the fuel is burned to drive a generator such as a turbine or used in a fuel cell. Typically, byproduct management involves at least the management of carbon dioxide. Management, as used here, means the storage, reaction, use or disposal of carbon dioxide in a manner consistent with the use of the AV. For example, carbon dioxide may be pumped out of the system, mechanically stored, or chemically stored by conversion to some other entity. Typically, current systems either store carbon dioxide as the compressed gas or pump the gas into the environment outside the AV. Either of these represents a parasitic energy loss to the system as a whole.
There are numerous conventional schemes to produce oxygen that might be used in an AV or other device requiring oxygen to provide power. The burning of sodium chlorate candles, electrolysis, or the thermal decomposition of hydrogen peroxide are known examples. There are also numerous methods of absorbing carbon dioxide or sulfur byproducts such as the use of methyl ethyl amine, lithium hydroxide or calcium oxide absorbents. Some submarines, for example, additionally employ a closed air system for diesel engines that removes carbon dioxide, normally using a chemical absorbent, thus allowing the production of electrical power without surfacing or snorkeling.
For use underwater, in an oxygen deficient environment or under other strenuous operating conditions, an AV must often possess a closed atmosphere. Oxygen, or its chemical precursors, can be loaded on the AV by a host vehicle which is usually manned. The sourcing of oxygen may be one of the greatest challenges for AV design.
The simplest and most straightforward oxygen source is compressed gas. For the purposes of comparison, an exemplary total AV energy requirement of 96.5 kW-hr is assumed. This represents the energy that may be required for a small platform unmanned vehicle to complete a proposed AV mission. Under this assumption and utilizing a proton exchange membrane (PEM) fuel cell employing reformed diesel as a fuel source (see below) as a model for calculating oxygen need, about 1270 moles or 40 kg of oxygen are required. At a storage pressure of 5000 psi, this represents a volume of approximately 83 liters, corresponding to two bottles, each 100 cm long, 25 cm in diameter, and made of 1.25 cm thick material. However, two tanks of this size preclude the loading of any other power plant elements in this specific AV environment. If the volume were reduced to one cylinder of this size, a pressure of 10,000 psi would be required which far exceeds the pressure handling properties of typical materials used in thin walled tanks.
Smaller diameter cylinders may use more effectively the limited volume available for the typical small platform AV power plant. Two cylinders, 17 cm in diameter and 100 cm in length, a size reasonable for integration into an AV, would need to be pressurized to over 12,000 psi to meet the needs of the mission. Again, materials are not currently available that would be satisfactory for this application. For 17 cm diameter tanks pressurized at 7500 psi, the tanks must be about 169 cm long. While this may be sufficient for some applications, mechanisms for further reducing the size of the oxygen source are desirable. While these size constraints may be specific for a particular application, the advantage inherent in reducing size and weight of the power plant in any AV is readily apparent to persons skilled in the art.
Chemical schemes for oxygen generation are well known. Oxygen has been routinely produced in the laboratory for many hundreds of years. Available methods for generation of oxygen include high temperature decomposition of inorganic compounds, thermal decomposition of organic compounds and thermal decomposition of hydrogen peroxide.
Laboratory oxygen production has been the subject of much scientific activity since the 18th Century. Work by Priestly, Scheel and Lavoisier provided many basic processes that produce this gas. Early techniques involved oxygen production by thermal decomposition, for example by heating an oxide of mercury, lead or manganese or potassium nitrate. These thermal decompositions all occur at elevated temperatures. Further, these oxygen precursors are not replenishable from the host vehicle used to launch AVs. Mercury and lead compounds are also not compatible with the tenets of atmosphere and material controls in a manned host vehicle. The process to produce oxygen by high-temperature thermal decomposition is also technically difficult in an AV environment.
Other metal peroxides and superoxides, such as those of barium, sodium, and potassium, are potential precursors for the oxygen needed by an AV power plant. These compounds have the same characteristics and inherent problems as the inorganic oxides listed above. Thus, they are not considered viable for use in an AV.
Chlorate compounds are also potential sources of oxygen. Of particular interest are sodium and potassium chlorate. These compounds thermally decompose to oxygen and the corresponding chloride at about 200° C. In the presence of a manganese oxide catalyst, the decomposition of potassium chlorate is driven to completion at 400° C.

Equivalent reactions occur for sodium chlorate, which is carried onboard submarines as “oxygen candles” for use in emergency situations. According to Equation 1, 1270 moles of oxygen are produced by 124 kg of starting material. On a weight basis, chlorate decomposition is a very attractive system. However, the chlorate decomposition reaction is very exothermic, although the temperature at which the thermal decomposition occurs, 400° C. (750° F.), is not exceptionally high. This heat energy may be used to pre-heat components for other reactions, such as, for example, combustion. The only energy required to initiate the reaction is a spark. The drawback to using this technology arises from difficulty in controlling the rate of oxygen production.
Reactions of iodates, metaperiodates, peraperiodates, nitrites, and hyponitrites may also be used to generate oxygen; however, the reactions involved are generally less suitable for use in an AV environment than the reactions described elsewhere herein. Thermal decomposition of oxygen rich organics may also have several common drawbacks. Organic compounds are generally volatile, and thermal decomposition might leave a carbon char in the AV that would require cleaning or extra process steps by operators between uses of the AV.
As mentioned above, carbon dioxide is a chemical byproduct of processes utilizing hydrocarbon fuels. Carbon dioxide is produced from hydrocarbons during burning or by conversion into other useful energy sources, such as, for example, hydrogen. Carbon dioxide is produced at a rate of about 70 moles per kilogram of hydrocarbon used. The approximately 1000 moles of carbon dioxide gas produced as a byproduct of the exemplary 96.5 kW hr supply occupies a volume of about 224,000 liters at standard temperature and pressure, which is clearly excessive in the AV environment.
Compression of carbon dioxide requires energy and a dedicated volume for storage. Energy can also be required to pump the gas away from the AV, which may not be possible in certain environments or under certain conditions such as, for example, a mission where non-detectability may be required or where carbon dioxide is not desirable in the surrounding environment. This parasitic energy load also lowers the overall efficiency of the power plant. Furthermore, using the limited volume of a typical AV for a dedicated tank or piping system can occupy volume needed for other functional elements. Clearly, compression or mechanical storage alone are not desirable, and other means for managing waste carbon dioxide are desirable.
Power generation schemes suitable for use in AVs are not limited to those that employ combustion of hydrocarbon fuels. For example, fuel cells, such as, for example, PEM and hydrocarbon direct fuel cells, present another viable alternative. However, use of fuel cells can present additional challenges.
First, hydrogen must be provided for the use of fuel cells. Storing and supplying hydrogen gas directly to the fuel cell is a simple method of providing the fuel. However, storage of hydrogen gas can often be impractical. For example, to meet the exemplary 96.5 kW-hr energy requirement with a 95% efficient PEM cell, approximately 2400 moles, or 5 kg, of hydrogen gas may be required. A 100 cm long 25 cm diameter tank typical for use in AVs can require a storage pressure of 20,000 psi for this amount of gas. This pressure far exceeds the material properties for current thin walled tank technology. If the hydrogen is distributed among three tanks, each tank 17 cm in diameter and 100 cm long, a geometrically advantageous arrangement for a specified application may allow for the inclusion of several tanks of oxygen, also required for the fuel cell. Although this arrangement may increase the available volume for storage of hydrogen, the pressure that would be required still exceeds 15,000 psi, which may far exceed the structural properties of conventional materials for thin wall tank design.
As an alternative to the storage of hydrogen, the gas may be generated onboard the AV. Metal hydrides have a very high weight relative to the amount of hydrogen produced and are thus unsatisfactory for this application. Alternatively, hydrogen may be supplied by the reformation of hydrocarbon fuel, such as, for example, diesel fuel. Reformation of hydrocarbon fuel can present even further challenges. First, any reformation system produces carbon dioxide in addition to hydrogen. Thus, the same challenges exist as for combustion of hydrocarbon fuels, i.e. management of waste carbon dioxide. Reformation also presents the additional challenge of separating hydrogen from carbon dioxide. The reformation of commonly available hydrocarbons also results in hydrogen gas that contains sulfur compounds. Because sulfur compounds can poison components of a fuel cell, they should be removed. Power systems considered for use in AVs that address this challenge are also desirable and not present in existing systems. Low sulfur hydrocarbon precursors may be utilized. These include, for example, synthetic fuels, and low sulfur diesel. These fuel sources may be of limited value because of higher costs and lower availability. Furthermore, use of these fuels presents the same challenges in terms of oxygen requirements and byproduct management. Finally, presently available reformer systems must be modified for use in the AV environment. This area is a source of continuing development.
Unfortunately, conventional oxygen generating systems have never been integrated with a carbon dioxide management function. It would also be desirable for an oxygen generator and/or carbon dioxide absorber to be able to manage sulfur. There is also a continuing need for energy generation systems suitable for use aboard AVs, particularly systems that address the storage and management of gases.